The present invention relates generally to ophthalmic characterization, and more particularly to mapping retinal thickness measurements to a movable measurement grid.
Diagnostics for eye disorders typically include a detailed ophthalmic examination of the retina. For initial examination, an eye doctor will view the retina through an ophthalmoscope. For a permanent record, the retina is typically photographed with a fundus camera. A fundus photograph directly records various anatomical features of the retina, such as the optic disc, fovea, blood vessels, and lesions. The imaging capabilities of fundus photography may be enhanced by supplementary techniques. A high-contrast image of retinal blood vessels, for example, may be photographed after the injection of a fluorescent dye into the bloodstream. The resulting image is referred to as a fluorescein angiogram (FA).
More sophisticated techniques have recently been developed for diagnostics of the eye. One such technique is three-dimensional optical coherence tomography (3-D OCT). In this technique, a light beam is directed onto the retina. Part of the beam is back-reflected. Interferometric analysis of the back-reflected light yields information on the structure of the retina. By varying optical parameters of the light probe, features at different depths below the surface of the retina may be probed. With this process, an image of a cross-section of the retina may be generated by scanning the optical probe along a line on the retina. By rastering the optical probe across the surface of the retina, a series of cross-sectional images may be produced. The series of cross-sectional images may be used to characterize the 3-D structure of the retina, and parameters such as local retinal thickness may be measured by 3-D OCT.
Analysis of the thickness of the retina may be used to diagnose diseases of the eye, for example, glaucoma and diabetic retinopathy. One indication of the health of the eye may be provided by comparing the retinal thickness of the patient's eye with reference data acquired from a population of healthy eyes. Progression of eye disease may also be monitored by measuring changes in retinal thickness over a period of time.
The retinal thickness is dependent on the loci (points on the retina) at which the measurements are made. The measurement loci are specified with respect to a reference point on the retina. Two common reference points are the center of the optic disc and the center of the fovea. One set of historical reference data from a large population of healthy retinas has been acquired with a Zeiss Stratus OCT 3, a commonly used instrument in the field of ophthalmology. This instrument measures the retinal thickness at loci on a circle centered at the center of the optic disc. The radius of the circle is fixed at 1.73 mm. Retinal thickness was also tracked in a large population of patients in the Early Treatment Diabetic Retinopathy Study (ETDRS). The center of the fovea was used as the reference point in that study.
For valid diagnostics, comparisons of the patient's data with the reference data must be determined at the same corresponding measurement loci. Errors will arise if the measurement loci in the patient's eye do not map properly to the measurement loci used for the reference data. A common assumption is that measurements by various instruments are properly referenced to the center of an anatomical feature such as the fovea. In some instances, this assumption may not hold, however. For example, the eye may move during examination. In some eyes, the fovea is off-centered in relation to the eye fixation point. Tracking progression of eye disease also requires that a patient's data is compared at the same set of measurement loci as a function of time. In addition, measuring retinal thickness in a neighborhood around a specific anatomical feature, such as a lesion, may be beneficial in diagnosing diseases of the eye.
What are needed are method and apparatus for mapping measurement loci to an anatomical feature and mapping retinal thickness values in a neighborhood around the anatomical feature.